Relative to other laser designs, diode lasers are more compact and robust, less expensive, electrically more efficient, radiate less waste heat, and easier to use as they do not require long warm-up times or great amounts of power (e.g., kilowatts) to operate. Overall, laser diodes offer a lower cost alternative for many applications. Until recently, however, diode lasers could not be used in products that require extremely high spectral stability and ultra-low wavelength drift due to strong temperature-dependence of the semiconductor material from which they are made. Single longitudinal mode diode lasers, such as distributed feedback (DFB) lasers, exhibit a temperature dependence of their optical emission wavelength of about 0.07 nm/°C. This temperature dependence alone makes the use of laser diodes difficult and costly in applications requiring a high degree of wavelength stability.
The use of volume holographic gratings, also termed VOLUME BRAGG GRATING (VBG), to stabilize the output wavelength of one or more diode lasers is known in the art as described, for example, in U.S. Pat. No. 7,889,776. VOLUME BRAGG GRATING and VBG are registered Trademarks of PD-LD Inc., Pennington, N.J.
FIGS. 1A and 1B represent side-view and top view, respectively, schematic illustrations of diode lasers of this type, often termed “hybrid external-cavity lasers” or HELLS in the art. In such lasers, a semiconductor gain section gain section, 110, provides optical gain. The optical radiation emitted from the semiconductor material (i.e., chip) diverges both perpendicular to and parallel to the epitaxial layer structure of the semiconductor gain section. The perpendicular direction is often termed the “fast axis”, as the radiation pattern in that direction diverges at greater angles than the divergence in the parallel direction, often termed the “slow axis”. Optics are used to collect and collimate the diverging beam. These optics are often termed a “fast axis corrector” (i.e., FAC), 120, and a “slow axis corrector” (i.e., SAC), 130. The FAC, 120, is typically located between the laser chip and the VBG, 140. The SAC, 130, may be located between the FAC 120 and VBG 140, as shown in FIGS. 1A and 1B. Alternatively, the SAC 130 may be located on the output side of the VBG (see for example FIGS. 2A and 2B, where the SAC 230 is on an output side of the VGB 240). FIGS. 2A and 2B represent side and top views, respectively of a HECL wherein the SAC is positions on the output side of the VRB. As the element of FIGS. 2A and 2B are substantially the same as those described with regard to FIGS. 1A and 1B, a detailed discussion of the elements of FIGS. 2A and 2B need not be further described.
Returning to FIGS. 1A and 1B, the reflectivity of the coating applied to the rear facet of the semiconductor gain section, R1, 111, located at a position z1 in FIG. 1, is high; typically 90-98% at the laser wavelength. The reflectivity of the front facet, R2, 112, is typically several percent or more. For example, U.S. Pat. No. 7,298,771 teaches a self-seeded HECL where the gain section operates as a laser and where the reflectivity of a front facet, R2, 112, may be in the range of 0.5-20% at the laser wavelength.
To a degree, a VBG-stabilized laser, shown in FIG. 1, may operate on a single longitudinal mode by using the VBG element as a partially reflective output coupler. The spectral reflectivity of the VBG element is substantially narrower than the width of the gain curve of the active medium of the laser. Only the longitudinal modes of the laser cavity with sufficient gain to exceed the lasing threshold will oscillate and be amplified. In some cases, the output of the laser module will consist of a single longitudinal mode.
In the conventional HECL shown in FIG. 1, the front and back surfaces of the collection/collimation optics. FAC and SAC, as well as the entrance and exit surfaces of the VBG 140 are anti-reflection coated so that the role of reflections from these surfaces does not contribute significantly to establishing optical cavities. The reflectivity of the wavelength selective feedback element (i.e., the volume Bragg grating of FIG. 1) is high relative to the reflectivity of the front facet of the gain chip, and thus a simplified analysis in which the front facet is ignored may be applied. In that case the functional optical cavity is established between the rear facet of the semiconductor gain section, at position z1, and the effective position of the VBG, which is determined by the length of the VBG, the refractive-index variations of the VBG, and the periodicity of the contained Bragg grating. The optical cavity established by these reflections is shown as having a length L2, 162, in FIG. 1, which defines a Fabry-Perot etalon. The spacing of transmission and reflection maxima produced by a Fabry-Perot etalon is:
                              Δ          ⁢                                          ⁢                      v            C                          =                  c                      2            ⁢                                                  ⁢            OPL                                              (        1        )                            where c is the speed of light in vacuum;        OPL is the optical path length.        
OPL is determined by the summation of the physical path length, Li, multiplied by the effective refractive-index, ηi, of each segment of the optical path.
In the HECL shown in FIG. 1, the semiconductor gain section having a physical length of 1.5 mm (millimeter) and a refractive index of approximately 3.5 (at a wavelength, λ0 of 1.064 μm), the effective length of the VBG may be approximately 1.5 mm with a refractive-index of approximately 1.5; the total thickness of the FAC and SAC may be approximately 2 mm with a refractive-index of approximately 1.5; and the total effective physical length of the Fabry-Perot cavity may be approximately 10 mm. The OPL is then approximately 15.5 mm, as shown in Table 1.
TABLE 1LiηiLi(mm)ηi(mm)Gain section1.53.55.25FAC + SAC2.01.53.00VBG1.51.52.25Free space5.01.05.00OPL15.50
The free spectral range of such a cavity, given by Eq. 1, is ΔνC approximately 9.7 GHz, or ΔλC approximately 37 pm (picometers) at λ0 =1.064 μm (micrometers).
The principle by which a conventional HECL operates is illustrated in FIG. 3.
As shown in FIG. 3, the semiconductor gain section, 110 (of FIGS. 1A, 1B) has a gain profile, 310 that is broad. Typically, the full-width at half-maximum of the gain profile (i.e., 3 db points) can be 30 nm (nanometers) or greater.
The laser cavity formed by R1, 111, and the VGB, 140, having a length of L2, 162, in FIGS. 1A, 1B supports many modes, indicated by the set of discrete modes 360 of FIG. 3. In conventional practice, the HECL is configured so that only one mode, 361, of the set of discrete modes 360, is at a wavelength for which the diode chip gain exceeds a lasing threshold, 365; such that a lasing output is achieved. The HECL will, preferentially, operate on that cavity mode. That is generate a lasing output at wavelength λ0.
Also shown in FIG. 3 is the spectral reflection profile, 340, of the VBG, 140 (of FIG. 1A). Although the spectral reflection profile, 340, of the VBG gain profile is shown as centered with respect to the gain profile 310, it will be appreciated that the spectral reflection profile, 340, of the VBG gain profile does not need to be centered with respect to the gain profile, 310, of the laser chip. Generally the VBG profile is often offset with respect to the laser chip profile. The width of the spectral profile of the VBG, ΔλVBG, 341, is considerably narrower than that of the diode laser gain profile, 310, and can be determined by the number of Bragg grating planes, N, formed in the VBG:ΔλVBG/λ0≈N  (2)
In a VBG having a length of approximately 3 mm, with Bragg grating planes spaced by λ0,/2n, where n is the refractive-index of the material, N may be of the order of 104, at λ0=1.064 μm. Thus, ΔλVBG is approximately 100 pm.
A Fabry-Perot resonator such as that formed by the R1, 111, and the VBG 140 may be further characterized by peaks in the transmission which correspond to cavity resonances within the etalon, and hence the allowed lasing modes of the cavity. A description of the transmission of light through a Fabry-Perot etalon is schematically depicted in FIG. 4 as curve 450. The transmission of light, T, can be expressed by:
                    T        =                                            (                              1                -                                                                            R                      1                                        ⁢                                          R                      2                                                                                  )                        2                                1            +                                          R                1                            ⁢                              R                2                                      -                          2              ⁢                                                                    R                    1                                    ⁢                                      R                    2                                                              ⁢                              cos                ⁡                                  (                                                            4                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      nL                                        λ                                    )                                                                                        (        3        )                            where n is the refractive index of the medium                    L is the path length,            λ is the wavelength of light,            R1 is the reflectivity of the rear facet of the resonant cavity, and            R2, in this case, may represent RVBG.                        
An exemplary HECL operating at λ=1.064 μm, with the laser chip having a rear reflectivity R1 of approximately 0.9 may have a VBG with a length of 3 mm and a reflectivity, RVBG approximately equal to 0.3. For the purposes of this simplified calculation, n=1, and L=15.5 mm. The resultant Fabry-Perot etalon has a finesse, i.e. a ratio of the free spectral range, ΔλC, to the full-width at half-maximum of the spectral distance between resonance, of approximately 4.8.
Referring to FIG. 4, a subset of the set of resonant wavelengths of the Fabry-Perot etalon is denoted as 460. The set of cavity resonances arising from reflections, from R1 and the VBG now are depicted as having finite width, and are shown as the shape 460 in FIG. 4. Superposed on the cavity resonances 460 is the spectral profile 441 of the emission reflected from the VBG, which is sufficiently narrow relative to the spacing of the cavity resonances so that only one cavity mode, 461, exceeds the gain threshold for lasing. The HECL will, therefore, oscillate at the wavelength of that cavity mode, λ0, 470.
Such operation, however, is not stable with respect to minor variations in operating parameters, such as variations in laser power, the temperature of the VBG, and thermal expansion of the optical cavity. In addition, instabilities often result from laser emission from the semiconductor gain section acting as a laser on its own. In the prior art disclosed here, HECL systems with VBGs are designed such that the semiconductor gain section is a laser. For example, in U.S. Pat. No. 7,298,771 the use of a laser diode in conjunction with a VBG, such that the reflected light from the VBG only causes a narrowing of the emission spectrum of the laser diode. This design has significant shortcomings, however, as the laser diode is operating without any reflected light from the VBG. The reflectivity from the Bragg grating simply narrows the existing laser emission. Thus, as the laser diode drive is varied to vary the output of the diode, instabilities may be introduced due to variations in spatial modes and gain saturation, leading to mode hops and linewidth broadening. The devices can even operate such that emission from the lasing of the semiconductor cavity occurs simultaneously with emission from the cavity formed by the VBG. An example of this mode of operation may be found in E Kotelnikov et al, Proc. of SPIE Vol. 8277, 2012. Similar effects can occur due to temperature variations.
The basic principles of operation of VBG-stabilized HECLs as described in the prior art are insufficient to guarantee single-longitudinal mode operation, in fact, relatively small values of the front-facet reflectivity, R2 (112 in FIG. 1A), result in resonances in the semiconductor gain section defined by its own Fabry-Perot cavity, independent of the HECL cavity formed by the VBG element. The value of R2 at which such self-oscillation occurs depends on the gain of the semiconductor gain section (and the value of R1), and can occur even at a reflectivity of a few percent or less. Furthermore, laser oscillation on multiple modes of the HECL cavity have been observed even when the value of R2 is as low as 0.5% for devices with long gain sections at high drive current. In such cases, the laser mode hops between allowed modes oscillating at different wavelengths (i.e., wavelength hopping).
Hence, a hybrid external cavity laser that provides substantially increased stability and reduced linewidth of generated laser light is needed in the industry.